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I. General Overview Figure 1: Photosynthesis & Carbon Cycling • Photosynthesis: II. Adaptations for Photosynthesis Figure 2: Leaf (C3) Structure Cuticle: waxy layer secreted by the epidermis that limits the amount of H2O loss from the leaf & entry of pathogens. • Epidermis (upper & lower): cuboidal cells that work in concert with the cuticle to limit water loss & prevent disease. Provides protection for the leaf’s delicate inner structure. • 1 Stomata: small openings in leaf’s underside. Allow the entry of CO2 into the leaf structure & the release of O2 & H2O (v). Enable the transport of water from roots to leaves. Found on the upper leaf surface in floating species. • Guard Cells: pair of cells that surround each stomate to regulate the amount of transpirational water loss from the leaf. When exposed to blue wavelengths in the morning, guard cells become permeable to K+ which serves to reduce the water potential (Ψ)of their cytoplasm. As a result, the guard cells take in water & become turgid, causing them to separate & open the stomates. • Figure 3: Guard Cells & Stomata (Daytime) Figure 3.1: Guard Cells & Transpiration Transpiration drives the upward movement of water molecules w/in the xylem tissue of the roots, stems, & leaves. This occurs because water molecules are joined by H-bonds & exert a pull on one another as they evaporate. If soil H2O availability is low, transpiration may lead to the desiccation of the plant body. • 2 Figure 3.2: Guard Cells & Stomata (Night & Water Stress) As plant cells lose water via transpiration during the night or during hot, dry conditions, they produce Abscissic Acid (ABA) that opens K+ exit channels. The mass efflux of K+ out of the guard cells raises the ψ of their cytoplasm, causing water to follow & the guard cells to become flaccid. This has the effect of closing the stomates to prevent continued transpirational water loss. • Spongy Mesophyll: loosely packed cells that form a network of air spaces to increase surface area for CO2 uptake & the loss of O2 & H2O (v). • Pallisade Mesophyll: columnar cells that act as the main site of photosynthesis, for they contain the highest concentration of chloroplasts within the plant (20-100/cell). • Figure 4: Chloroplast (Pallisade Mesophyll) Chloroplast are *Plastids that contain the photosynthetic pigments (mainly chlorophylls) for the conversion of sunlight to chemical energy in the form of sugars. a) Descended from relatives of modern Cyanobacteria (“blue-green algae”). b) Consists of an inner & an outer membrane. The inner membrane encloses a fluid-filled space called the Stroma, which contains enzymes for producing carbohydrates from CO 2 & H2O. c) Suspended in the stroma are membranes that form a set of interconnected disk-like sacs called Thylakoids. Thylakoid membranes contain chlorophyll & other photosynthetic pigments. d) Thylakoid membranes, like the inner mitochondrial membrane, are involved in ATP synthesis. Thylakoids are organized into stacks called Grana. • *Plastids function in pigment production (chromoplasts) as well as sugar synthesis (chlor0plasts) & starch storage (leucoplasts). Vascular Bundle: contains conducting tissue that is continuous with the stem. Xylem serves to bring water & dissolved solutes up to the leaf tissues & Phloem transports sugars to other parts of the plant. • 3 III. Properties of Light Figure 4: Electromagnetic Spectrum EM Spectrum consists of all forms of radiation that travel in waves that are differentiated from one another by their respective Wavelengths (λ). • The visible portion of the electromagnetic spectrum ranges from wavelengths of 380 nm (violet) to 760 nm (red). Visible light is composed of small particles called Photons. The energy of a photon is inversely proportional to its λ. • Photosynthetic Pigments • Substances that absorb specific λ’s of visible light are Pigments. The ability of a pigment to absorb various wavelengths of visible light can be measured by placing a solution of the pigment in a Spectrophotometer. Figure 5: Measuring Absorption: Spectrophotometer 4 Figure 5.1: Absorption Spectrum of Chlorophyll Pigments a & b Blue • Red Absorption Spectrum: a) The absorption spectrum for the chlorophyll pigments suggests that they most effectively absorb blue & red visible wavelengths. Figure 5.2: Action Spectrum of Photosynthesis: T.W. Englemann Blue Red Quantity to be Measured: Method of Measure: Results: a) The distribution pattern of the aerobic bacteria around the spirogyra represented the first Action Spectrum for the photosynthesis. This pattern illustrates how photosynthetic rate is dependent on specific λ’s of visible light. b) When Englemann compared the action spectrum of photosynthesis with the absorption spectrum for the chlorophylls, they very nearly matched, suggesting that chlorophyll played a vital role in driving photosynthesis. 5 • Action Spectrum: Figure 5.3: Chlorophyll Absorption Spectrum vs. Photosynthesis Action Spectrum The action spectrum does not quite match the absorption spectrum for chlorophylls because secondary pigments called Carotenoids absorb certain other visible wavelengths. Their presence increases the variety of visible wavelengths that can drive photosynthesis. • Figure 6: Chlorophyll Structure & Excitation • Chlorophyll: 6 The types of pigments in the thylakoid membrane include: a) Chlorophyll a: contains a methyl group (-CH3) on its porphyrin ring. Instrumental in the light reactions that convert light energy into chemical energy. Receptive to wavelengths of 662nm (red) & 430nm (blue). b) Chlorophyll b: contains a carbonyl group (-CHO) on its porphyrin ring. Does not participate in the light reactions directly, but rather passes photons to chlorophyll a. Receptive to wavelengths of 642nm (red) & 453nm (blue). c) Carotenoids: along with chlorophyll b, serve to channel photons to chlorophyll a. Where as chlorophyll a & b absorb the same wavelengths, carotenes (hydrocarbons) & xanthophylls (oxygenated carotenoids) are able to absorb wavelengths that chlorophyll a & b cannot, thus broadening the variety of wavelengths that can drive photosynthesis. • IV. Chemistry of Oxygenic Photosynthesis General Formula 6CO2 + 6H2O C6H12O6 + 6O2 (∆G = +686 kcal/mol) • H2O (electron source) is completely oxidized to O2 in the presence of light. CO2 is reduced to the sugar PGAL. Light Dependent Reactions Figure 7: Photosystem (Thylakoid Membrane) *Antenna pigments surround & channel photons to the reaction chlorophyll a molecule. Two photosystems, named for the order of their discovery, function during the light reactions of photosynthesis: a) Photosystem I: consist of an antenna complex surrounding a reaction center chlorophyll a complex that absorbs maximally at wavelengths of 700nm (P700). b) Photosystem II: consist of an antenna complex surrounding a reaction center chlorophyll a complex that absorbs maximally at wavelengths of 680 nm (P680). • • Light Dependent Reactions: • Photosystem: 7 Figure 7.1: Light Reactions: Noncyclic Electron Flow Figure 7.2: Light Reactions: Noncyclic Electron Flow (Thylakoid View) *Notice that both forms of chemical energy (ATP, NADPH) formed during noncyclic electron flow collect within the stroma of the chloroplast, the site of the Calvin-Benson cycle. 8 • Noncyclic Electron Flow: a) Upon absorbing 2 photons, the antenna complex of photosystem II channels them to the reaction center chlorophyll (a) molecule, P680. By absorbing these photons, 2e- in P680 jump to a higher energy state, whereby the molecule becomes Photooxidized. These e- are captured by a Primary Electron Acceptor w/in the thylakoid membrane. b) The photooxidation of the reaction center makes it a powerful oxidizing agent -during a process called Photolysis, an enzyme “splits” water into ½O & 2H+, using its 2e- (from oxygen) to replace those lost by P680. c) The donated e- travel along an Electron Transport Chain embedded in the thylakoid membrane. The e- cascade down the chain & establish a proton gradient by transporting H+ from the stroma into the thylakoid space. d) The proton gradient stores a great deal of potential energy, which is released as they rush back into the stroma via ATP Synthases (chemiosmosis). As a result, ADP is Photophosphorylated to ATP. For every H2O molecule “split”, 1 ATP is produced via photophosphorylation, resulting in 12 total ATP’s. e) Simultaneously, P700 receives 2e- from Photosystem II to replace those lost to its primary electron acceptor. The donated e- are passed to the enzyme NADP+ Reductase, which stores them in the coenzyme NADPH. For every H2O split, 1 NADP+ is reduced to NADPH, for a total of 12 NADPH’s. f) End Products: 12 NADPH, 6O2, 12 ATP (NADPH & ATP are used during the Calvin Cycle to reduce CO2 to sugar). Figure 7.3: Light Reactions: Cyclic Electron Flow • Cyclic Electron Flow: a) May result when ATP is consumed at a high rate by the many reactions occurring in the stroma, leading to an ATP deficit. Consequently, NADPH will begin to accumulate as the Calvin Cycle slows down (requires 18 ATP’s). This rise in NADPH may stimulate a temporary shift from noncyclic to cyclic electron flow until ATP supply catches up with demand. 9 Sugar Production Figure 9: Calvin-Benson Cycle • Calvin-Benson Cycle: a) During Carbon Fixation, 3CO2 are incorporated into the skeleton of the 5-carbon sugar RuBP via the enzyme Rubisco, forming 3 molecules of a 6-carbon compound. b) The 6-carbon compounds break down to form 6 molecules of the 3-carbon compound 3-phosphoglyceric acid. For this, the Calvin Cycle is also known as the C3 Pathway & plants that initially fix carbon this way are C3 Plants. c) 3-phosphoglyceric acid is phosphorylated via the hydrolysis of 6ATP to form 6 molecules of 1,3-diphosphoglyceric acid, which is reduced by 6NADPH to form 6 molecules of the 3-carbon sugar PGAL (aka G-3-P). d) In order for the cycle to continue, 5 of the 6 molecules of PGAL are recycled to make more RuBP for the carbon fixation of additional CO2 –phosphorylated by 3 ATP. e) Since 1 molecule of PGAL is equivalent to half a glucose molecule, another 3 CO 2 must enter the cycle to generate another 6 PGAL’s. The 2 net PGAL’s generated by this represent the major products of the Calvin-Benson Cycle. Fate of Photosynthetic Products • Once formed, PGAL can be utilized by the plant in the following ways: a) 50% of the PGAL will be converted into simple sugars that will be consumed during cell respiration to form ATP. b) Some of the PGAL will be used to form the glucose required to form cellulose, the major structural polysaccharide within plant cell walls. c) Some of the PGAL will be modified to form other vital macromolecules (nucleic acids, lipids, amino acids, etc). d) Excess PGAL will be converted to form starches to be stored mainly within the root system. *All PGAL that is transported through the plant body via phloem tissue is first converted into the disaccharide sucrose. 10 V. Alternatives Mechanisms of C-Fixation Figure 10: Conditions for Photorespiration Under prolonged hot, arid conditions, guard cells lose turgidity & begin to close, preventing continued gas exchange. As a result, CO2 levels within the leaf’s spongy mesophyll drop as it continues to be consumed for Calvin-Benson & the levels of O2 by-product rises. • Under these conditions (high O2, low CO2), the enzyme RUBISCO fixes oxygen & introduces it into the Calvin-Benson cycle. This leads to a metabolic pathway known as Photorespiration … • Figure 10.1: Photorespiration Pathway Like cell respiration, this pathway consumes O2 & releases CO2 (hence the term “photorespiration”). However, this pathway does not result in the formation of sugar (PGAL) & is thus seemingly wasteful. The benefit of photorespiration under hot & arid atmospheric conditions is that RUBISCO acts as an antioxidant to reduce the amount of free oxygen within the cell that can lead to the formation of reactive oxygen species (ROS) … • 11 Figure 10.2: Antioxidant Function of RUBISCO • Photorespiration: a) Although “wasteful” in the sense that it results in no PGAL produced, the need for photorespiration to prevent ROS production outweighs the need for sugar production under prolonged hot & arid conditions. b) Plants other than C3 plants enjoy the “best of both worlds” under these conditions, being able to continue sugar production even though CO2 levels drop & O2 levels rise within the leaf tissue … Figure 11: C4 (Hatch-Slack) Pathway • C4 Plants: a) In contrast to C3 plants, C4 plants (sugar cane, grasses) possess leaves in which the palisade cells completely surround those of the bundle sheath. This allows for the spatial separation of carbon fixation & the rest of the Calvin-Benson cycle. 12 b) Carbon fixation initially occurs in the mesophyll cells in which CO2 is added to PEP (starting material) via the enzyme PEP Carboxylase to form oxaloacetate & eventually malate (represents stored CO2). Compared to rubisco, PEP carboxylase has a much higher affinity for CO2 relative to O2. As a result, it can fix CO2 instead of O2 even during times when the stomata are closed & CO2 levels are too low for rubisco to be effective (avoids photorespiration). c) After CO2 fixation, the mesophyll cells export malate (stored CO2) to the bundle sheath cells through plasmodesmata. Within the bundle sheath cells, the malate is degraded to release large amounts of CO2, which establishes a high CO2 to O2 ratio favorable for RUBISCO to introduce CO 2 into Calvin Benson ( PGAL) & avoid photorespiration. Figure 11.1: C4 vs CAM Pathway *CAM plants include succulent (fleshy) plants that store a large amount of water in their tissues (cacti, jades, etc). These plants live in environments so hot & arid, that stomates can only be opened for gas exchange at NIGHT! • CAM Plants: a) In contrast to C4 plants, CAM plants exhibit a temporal separation of carbon fixation & Calvin-Benson (c-fixation & Calvin-Benson occur at different times). b) During the night, with their stomata open, they take up CO2 & incorporate it into a malate (stored CO2), a mode of carbon fixation known as Crassulacean Acid Metabolism (CAM). c) The mesophyll cells of CAM plants store the malate in vacuoles until morning, when the stomata close. During the day, when the light reactions can supply ATP & NADP for the Calvin Cycle, CO2 is released from the malate made the night before to establish a high CO2 to O2 ratio so RUBISCO can fix CO2 which becomes reduced into sugar (PGAL). *Although both pathways consume a greater amount of ATP in order to initially fix CO 2, this additional investment is outweighed by the increased sugar output gained by avoiding photorespiration. 13